We saw in the last section that most observers in the electronics industry would rate the lead-free solder issue as their environmental priority. In this section, we are going to explore the rationale for removing lead, and some of the alternatives available, before focussing on the likely lead-free solder solutions and the implications for the industry.
Lead has long been known to have cumulative harmful effects: adults exposed to lead can suffer difficulties during pregnancy, high blood pressure, nervous disorders and memory concentration problems; children exposed to lead can suffer from damage to the brain and central nervous system, slow growth, hyperactivity, and behavioural and learning problems. ‘Exposure to lead’ may occasionally take the form of exposure to lead fumes in high temperature processing (higher than used in the electronics industry), or of accidental oral ingestion (handling food with hands contaminated by lead-containing solder pastes). However, most exposure comes from small quantities of lead in the air we breathe and in the water we drink – lead-containing materials anywhere in the water supply chain will dissolve slowly, especially in soft-water areas.
Once in the body, lead is distributed through the blood stream and is absorbed first in soft tissue and eventually in bones, where it will remain for long periods of time. Lead can thus build up as a result of continued exposure to even quite small amounts in the environment. Significantly, children and developing foetuses are known to absorb lead more readily than adults.
For water distribution systems, alternative pipe materials have been introduced,
first copper, and more recently plastic. This process has for some years attracted
considerable public subsidy to retrofit older properties. Lead-containing plumbing
solders have also been replaced.
There has also been environmental pressure to remove lead from a wide range of products from paint to fishing weights. Remember that the problem is not purely historic: in July 1999, the EPA claimed that lead poisoning still adversely affected the health of almost one million children a year in the USA.
Before reading on, think about a range of different types of electronic assembly, and try and identify as many applications as possible where lead would normally be a constituent.
Hint: if you search for "lead in electronics" you will come up with a manageable number of hits (low hundreds) which will yield some applications, although there is a preponderance of material on lead-free solders. In addition you will find many comments on the drive to eliminating lead, much of it from US sources and severely critical.
Within the board assembly environment, there are obvious implications for safe material handling practice and the disposal of waste. In wave soldering, the removal and disposal of dross presents a particular hazard; in reflow soldering, operators may come into contact with both waste solder paste and paste residues on containers and wipes.
However, as regards the external observer’s view of the electronics industry, the main problem relates to the end-life disposal of equipment. Although the electronics industry is responsible for only around 0.6% of world lead usage, disposal of the end-product is uncontrolled. This means that much of it will end up as land-fill, from which lead may eventually dissolve in water and has the potential to end up in the water distribution system. Pressure to remove lead from the environment has therefore targeted electronic waste as a potential contaminant:
Legislation has been a driver: in 1991 the US Senate proposed a Lead Exposure Reduction Act (‘the Reid Bill’), which would have put a 0.1% limit on the lead content of products. Although both this and a 1993 follow-up bill were mothballed, a threat remained, and some research work was initiated.
In the USA the threshold for reporting on lead under the EPA Toxic Release Inventory has now been reduced to only 10lbs (4.5kg) annually, but the main impetus is currently coming from the EU WEEE Directive mentioned in the previous section. In order to prevent lead entering the waste stream, it has been agreed to ban the use of lead metal in electronics from 1 July 2006. However, even more significantly for many companies, the main driver will be competitors using a ‘lead-free’ claim as a product differentiator, to assist in marketing.
Iwona Turlik reported (Electronics and the environment, IPCWorks Proceedings, October 1999) that in 1983, 66% of consumers would switch to an equivalent brand with a ‘better’ environmental impact; in 1998, the number had risen to 76%.
The Japanese industry first presented a lead-free roadmap in January 1998, when major electronics companies declared their own intentions to reduce lead use substantially within a year or two and to eliminate all lead from major products by 2001. Panasonic took the lead in marketing products as lead-free and ‘Produced for the Environment’: their first lead-free mini-disc player, launched in October 1998, gained 11% market share on release.
Progress since has not been quite as fast as anticipated, and the July 2002 plan was for the total abolition of lead solder on the following schedule:
by end of FY2002
Before we look at removing lead from board and components, we suggest that you review the CRT recycling issue. A good starting point would be the Materials for the Future Foundation web site.
How big is the problem? How severe is the problem? How do the different methods for dealing with the problem compare? Are there any general learning points from this?
Whilst CRT recycling is a major challenge, the lead in glass is very tightly bound, and in most landfill situations will not give rise to dissolved lead. However, the lead elsewhere in electronic assemblies, which is in metallic form, is more available to contaminate the environment, so we are faced with a legislative requirement to make electronics lead-free. In order to do this, we have to tackle lead in the board finish, and the components, as well as the interconnection medium.
ICAs are so called because they conduct equally in all directions, and are mixtures of metal in fine powder form in a polymer base. The conductive path is formed by contact between metal particles after curing. Typical ICAs are silver-loaded one-part epoxy compounds supplied uncured.
ICAs are applied in the same way as conventional solder paste, but cured rather than reflow. There is, however, no behaviour similar to solder reflow, because the surface tension forces are entirely different.
ICA properties vary with the percentage of metal, being limited by the viscosity of the resulting compound, as with solder paste. Compared with unloaded resin, they exhibit significantly increased thermal conductivity, but the electrical conductivity values for silver and copper-loaded resins are several orders of magnitude below those for the metals themselves.
ICAs have been evaluated as potential replacements for lead-containing solders. Unfortunately, whilst most adhesives form a stable junction with precious metal-coated components, most are incompatible with the base metal finishes found on SMDs and boards!
Alpha Metals have developed materials that reportedly exhibit greatly increased stability under heat and humidity ageing using an adhesive formulated to encourage oxide penetration by small conductive particles as the polymer hardens and shrinks, as shown in Figure 1. You can read more about this material at this link.
The main problem with any kind of conductive adhesive is the stability both of the bonds between particles and between particles and contact surfaces, and of the resin itself. This is particularly the case at elevated temperatures, where most of the resins used will creep and lose adhesion. Much work has been carried out to improve performance by stabilising the resin and, by including anti-oxidants, to reduce the rate of build-up of oxides on the contact surfaces within the joint structure.
Note: Whilst the use of ICAs for solder replacement is a major challenge, be aware that silver-loaded resins have been used for chip attach for over 30 years, proving highly reliable in making joints between non-fusible surfaces.
ACAs were developed in the 1950s as dielectric coatings with randomly dispersed particles which made electrical connections only in the vertical direction. Such ‘Z-axis’ bonding films have been used since the 1980s to make connections to Liquid Crystal Displays.
ACAs have a low loading of conductive particles, so that individual particles do not contact one another and there are no unwanted electrical paths in the X-Y plane. When sandwiched between conductors, non-conductive polymer is squeezed out, allowing a mono-layer of conductive particles to bridge the gap, as shown in Figure 2.
With particles typically less than 25 µm in diameter, non-coplanarity must be very tightly controlled. For this reason, ACAs generally perform best when one adherend is compliant, and have thus found particular favour with users of flexible printed circuits. The coplanarity problem can be reduced by using compressible conductive particles such as metal-coated elastomeric spheres which deform under bonding pressure, an action which also helps maintain force on the pressure contacts forming the junctions.
The main problem with ACAs lies in their method of use, because pressure must be applied until curing is complete. For this reason, ongoing developments in ACAs include replacing pressure contacts by particles which bond to the substrate, as well as placing particles in an array, rather than at random, to enhance electrical performance.
The performance limitations and practical difficulties associated with adhesives suggest that the most likely outcome is the further development of lead-free solders.
Of course, when selecting alternative technologies, one has to be careful not to end up in an even worse situation! We have to take into account such issues as:
Concerning this last point, keep in mind that the tin-lead system has been researched for many years and is very well understood.
For most board assembly purposes, it would be desirable to have a ‘drop-in replacement’ for the commonly used eutectic tin lead alloys. Such a material would have a melting point under 200°C, be able to be reflowed at under 230°C, have a small pasty zone and acceptable physical, electrical and thermal properties. The material would also be compatible with other metallisation materials, exhibiting acceptable wetting, and be available without using exotic materials and at a similar price.
The extent to which price is important will depend on the application: for wave soldering, a price similar to tin-lead is important because the price of the commodity purchased is closely related to the price of the raw materials and a large mass of solder is needed to fill the solder pot; for reflow soldering, the base metal is only a comparatively small percentage of the final paste cost.
Unfortunately, such a drop-in replacement with this unique combination of properties doesn’t exist! A wide range of alternative solder materials has therefore been explored, if not yet fully evaluated. The only viable alternative found has been to replace the bulk of the lead by tin, and the search for materials becomes what Harrison called “the search for alloying additions to tin to offer the best, or failing that the least worst, match to the characteristics of the tin-lead family as electronic solders”.
Most of these alloys have additions of zinc, indium or bismuth:
1 With its major source being Russia, palladium is another case of a material whose price has fluctuated over a 7:1 range over recent years due to economic and political forces, with the result that its use in lead-frame plating has not taken off to the extent originally predicted.
Any alternative solder must be capable of being made in bar form for use in solder wave, be able to be drawn into wire for hand soldering operations, and be available as powder for solder paste manufacture. Note that not all alloys can be extruded and drawn, nor can they effectively produce uniformly spherical, oxide-free powders.
The 1999 NPL Report Lead-free soldering: An analysis of the current status of lead-free soldering, divided the broad spectrum of alternative materials into four categories:
This means that the lead-free solder substitutes evaluated for printed circuit assembly will come from the range of materials melting at 200–230ºC. These often have three or four components, with tin plus silver as the most common base, with additions of copper or bismuth. As is inevitable with such developments, some materials are subject to patent and licence restrictions.
The favoured materials are:
2 When a solder formulation is expressed in this way, it means that the first element named forms the balance of the composition. In this case, the alloy contains 99.3% tin and 0.7% copper.
3 Although tests by Nortel Networks indicated that tin-copper performed better than tin-lead under thermal fatigue testing, Biglari and Oddy found that fatigue life for this alloy was 50–60% of that for Sn-40%Pb.
The Brite-Euram project found that SAC actually performs better than tin-lead, and that the addition of 0.5% of antimony (SACS), made particularly for wave soldering purposes, strengthened the alloy and further increased reliability. Castin™ is a patented alloy which melts in the range 217–220°C, and has a broadly similar composition (Sn-2.5%Ag-0.8%Cu-0.5%Sb)4. Unfortunately, some sources report concern about the toxicity of antimony.
All these materials have melting temperatures 30–40°C higher than eutectic tin-lead. Fortunately, the general conclusion from the IDEALS project5 was that, both for wave soldering and reflow, the temperature does not have to be increased pro rata to the solder melting point, although the increased thermal input still has to be supplied by combining higher temperatures with longer process times.
4 A full report on this is downloadable at http://www.aimsolder.com.au/pdf_misc/AIM CASTIN Booklet.pdf.
5 Information on this project, and a number of other interesting articles on lead-free issues are available at http://www.alphametals.com/products/lead_free/tech_art.html.
The critical question with reflow is by how much the temperature will need to be increased, compared with normal conditions for tin-lead solders. The indication is that the critical feature is the integral of the time over liquidus temperature, and that equally successful results can be achieved either by using a short spike or a more extended exposure at a lower temperature. In Figure 3 these are referred to as ‘angle’ and ‘hat’ profiles.
It seems likely that a peak temperature in the 240–250°C region will be necessary for SAC, though Suraski reported that SAC reflowed just as well with a 230°C spike. However, success under such conditions was critically dependent on the equipment, on the firing atmosphere, and on the flux used.
Wave soldering with lead-free materials has been somewhat more problematic than for reflow, and has been further complicated by the parallel drive to carry out the process with VOC-free fluxes. This is an aspect to which we will return in the next section. The indication is that the process is feasible with high yield, although the process window is relatively narrow.
The problem is one of cost, and potentially needing to use silver-containing solders. The conclusions from the IDEALS project were that “the melting point of SnCu is too high for wave soldering at acceptable temperature levels”, and Biglari and Oddy recommended using eutectic Sn-3.8%Ag-0.7%Cu-0.25%Sb (SACS) for general-purpose wave soldering, and non-eutectic Sn-4%Bi-1%Ag-2%Sb (SBAS) for single-sided wave assembly.
However, it has been reported that Matsushita have produced several million VCRs with a paper-phenolic board wave-soldered with the nickel-stabilised Sn-0.7Cu lead-free alloy patented by Nihon Superior Co. The nickel addition has the effect of making it possible to achieve bridge-free wave soldering with a solder bath temperature of around 255°C, which is within the range that paper-phenolic can handle for the 3–4 seconds it takes for any part of the board to pass through chip and laminar waves. This material has also been used in both horizontal and vertical systems to produce a lead-free HASL finish.
So clearly, whilst the situation for reflow is fairly well researched, you will have to take account of different process preferences among your assemblers when you move towards lead-free assembly.
Apart from needing to increase the primary process parameters of temperature and time, and allow for the process window being somewhat narrower than for tin-lead, what else can we conclude from the research work into lead-free solders? A review of the literature indicates that:
Before reading the rest of this section, think back on what you know about the properties of lead-free materials, and put on paper the implications that you think these might have for fabricator, assembler and designer. If you get stuck, try a Google search using the terms "lead-free" +"implementation issues".
Now read on, and review your answer against our comments.
A major impact will result from the higher process temperature that will be needed for reflow:
Lead-free materials, processed at the minimum temperature in order to reduce the damage to components, will have a narrower process window than eutectic tin-lead. Specific problems which have been reported are:
There is likely to be a general problem with rework, in ensuring compatibility between the materials used. Certainly if conventional lead-containing solders have been used, it may be prudent to repair using the same materials. The danger of mixing these with lead-free materials is not just that one might create a hazardous low-melting phase, such as that reported with bismuth alloys, but the resulting solders may have extended pasty ranges and be difficult to work with.
From the design perspective, the most effective action will be to indicate in some clear way that a board has been designed to be assembled with lead-free solder. The move to lead-free must also be communicated clearly within the whole of the supply chain.
We have already indicated the need for different board finishes, with the elimination of conventional HASL. However, substituting lead-free HASL for the current material leaves us with a full range of possible materials, as was discussed in AMI4812 Materials and Processes for EDR. Of course, all of these will be subjected to higher temperatures in use, and minor modifications may be needed to accommodate lead-free processing.
Apart from issues of solderability, the main problem is high-temperature leaching of copper from both lead-frames and pads. It is particularly important to keep temperature at a minimum. It is reported that wave soldering at 265°C can remove 10µm of copper – only 10% of the lead-frame thickness, but one-third of a typical copper pad.
The higher time at temperature will affect component choice and specification, and was originally predicted to result in the demise of the SM aluminium electrolytic capacitor, although versions being developed can now withstand 250°C. JEITA have however warned that there may be some mismatch between the expectations for temperature profile by equipment makers and components makers, the former likely to assume the ‘hat’ type of profile shown in Figure 3, and the latter the ‘angle’ type. The danger is that a component may be able to withstand a high temperature for a short time, but less able to survive longer exposure, albeit at a slightly lower temperature. The specification of components is certainly an area where care is needed.
Handwerker expressed concern over plastic-moulded packages that higher reflow temperatures “will have a severe, negative impact on component performance and, therefore, on the component ratings”. Certainly it has been reported that absorbed moisture, leading to popcorning, is a worse problem at the higher temperatures needed by lead-free systems. Problems have also been reported with LEDs damaged by a combination of heat and mechanical stress, where the epoxy softens, allowing inner parts of the assembly to move if any residual stress remains as a result of the assembly operation.
At least in the short term, the availability of components with lead-free terminations continues tobe a constraining factor, although incompatibility is only important where bismuth-containing solders are to be used. For packages with lead-frames, nickel-palladium with a gold flash has historically been the most common substitute for solder plating. This has 1–3 µm plating of nickel, followed by a 0.02–0.15 µm layer of palladium, and a 3–10 nm gold flash. The palladium protects the nickel from oxidation; the nickel keeps the base metal in the lead frame from diffusing into the palladium; the gold improves wettability. Texas Instruments pioneered the use of this material in the late 1980s, but its slow uptake has been due to supply and price issues, most of the world supply coming from Russian and South Africa. There is also competition from palladium use in catalytic converters for vehicles.
A simpler option would be a pure tin finish, a return to the dominant finish in the 1960s. However, there are concerns about the potential for the growth of tin ‘whiskers’, which grow from the surface of the stressed metal. There is more about this in AMI4812 Materials and Processes for EDR. Whisker growth can be reduced by adding lead (not very helpful!), and alternatives being investigated are high-tin alloys with bismuth, silver and copper. This is an area where solutions are still being sought.
A number of area packages such as BGAs use solder ball terminations, and it seems likely that these will migrate to SAC. However, this will not work for some complex devices, such as CBGA parts, which use a high-melting solder to define a sufficiently high stand-off from the substrate in order to enhance reliability.
Tin and lead are a unique combination, in the sense that varying the percentage
of tin produces useable solder alloys whose melting temperatures range from
183°C to over 300°C. This has led to their use in component manufacture
and similar processes that require a differential between melting points.
One example where solder is used inside a component is for die attach in power products, where the bonding material generally needs to have both high thermal and electrical conductivity. The most common solders for this application are either high-lead (such as Pb-5%Sn, Pb-5%Sn-2.5%Ag, Pb-2.5%Ag-2%Sn) or high-tin (Sn-25%Ag-10%Sb, and Sn-8.5%Sb). Whilst the two lead-free alloys in this list are used extensively, they have solidus temperatures of only 228°C and 236°C respectively. This creates a problem when parts are to be assembled with lead-free solders, and are specified for reflow at 245°C for extended periods!
There are as yet no straightforward inexpensive solutions: a Japanese group reported work on an Al-Zn-Mg-Ge alloy, but their results were not encouraging; known alternatives include high-Au alloys such as Au-20%Sn and Au-3%Si, but these are considerably more expensive than high-Pb alloys, and are arguably too rigid for some applications.
The search continues at NCMS among other places, for a solder that will have a high melting point yet be free of both lead and precious metals. Encouragingly, although no details have been published, HOTSOL, a “High Operating Temperature Solder with Zero Lead”, has been the subject of a UK Smart Award and it is currently being evaluated in a pan-European project. Watch http://www.tcore.co.uk/Research/Solders.htm for details.
In Chip Scale Review in September 2001, Vern Solberg of Tessera made the comment that “As far as any health risk is concerned, most experts believe the lead-free issue is more political than real”. Those who have listened to the screams of anguish on various IPC forums will know that there has been considerable reluctance to accept the move to lead-free, a reluctance supported by lack of any cogent engineering analysis to provide a foundation for the environmental concerns.
However, although Solberg may well be right in identifying the driver as being political rather than technical, the reality is that lead-free will affect our industry, whether we like it or not. We therefore need to understand the implications of lead-free soldering, and prepare to embrace it, even though the change may result in our producing products that are less reliable than formerly. Given the implications for both joints and components, the EDR professional has a key role to play in ensuring that the change to lead-free is managed successfully and at minimum risk to the company.